Secondary battery of improved lithium ion mobility and cell capacity

ABSTRACT

Provided is a lithium secondary battery having improved discharge characteristics in a range of high-rate discharge while minimizing a dead volume and at the same time, having increased cell capacity via increased electrode density and electrode loading amounts, by inclusion of two or more active materials having different redox levels so as to exert superior discharge characteristics in the range of high-rate discharge via sequential action of cathode active materials in a discharge process, and preferably having different particle diameters.

FIELD OF THE INVENTION

The present invention relates to a secondary battery with improvedlithium ion mobility characteristics and increased cell capacity. Morespecifically, the present invention relates to a secondary batteryhaving improved discharge characteristics in a range of high-ratedischarge without degradation of general characteristics of the battery,by fabricating a cathode using two or more active materials havingdifferent oxidation-reduction (hereinafter, simply referred to as“redox”) levels so as to exert superior discharge characteristics in therange of high-rate discharge via sequential action of cathode activematerials in a discharge process, and having maximized cell capacity viaincreased electrode density and loading amounts.

BACKGROUND OF THE INVENTION

With recent development of mobile communication and theInformation-Electronic Industry, higher capacity, smaller and lighterlithium secondary batteries are increasingly in demand. However, withdiversification of functions of the portable or mobile electronicequipment, which is thereby concomitantly accompanied by increasedenergy consumption of the equipment, there is also a strong need forrealization of higher power and capacity of the batteries. Therefore, agreat deal of research and study has been widely conducted to increaseC-rate characteristics and capacity of the battery cells.

However, there is the presence of reciprocal relationship between C-ratecharacteristics and capacity of the battery cell. That is, when aloading amount or electrode density of the cell is increased in order toimprove cell capacity, this attempt usually results in deterioration ofC-rate characteristics of the battery cell.

Upon taking into consideration ionic conductivity of active materials,lithium secondary batteries, as shown in FIG. 1, are needed to maintainthe electrode porosity over a predetermined level. Whereas, if theelectrode is rolled at a high-rolling reduction rate in order to achieveincreased loading amount or electrode density, the electrode porosity isexcessively decreased, as shown in FIG. 2, which in turn leads to arapid decrease in the C-rate. Further, when the same active materialshaving different particle diameters are used as an electrode activematerial, it is possible to accomplish a high electrode density bymoderate rolling, but the electrode porosity is strikingly decreased asshown in FIG. 3, thereby leading to significant decreases in the C rate.

Therefore, although it is important to maintain appropriate porosity inorder to meet a proper level of C rate characteristics, thethus-maintained void remains as a dead volume where the electrode isfree of the active materials.

Secondary batteries must maintain a given level of C-rate suited for thecorresponding uses thereof. In particular, secondary batteries for usein electrically-driven tools that require elevated power or secondarybatteries for use in electric vehicles (EVs) and hybrid electricvehicles (HVs) require significantly higher C-rate. Consequently, inorder to increase the battery power, there is a strong need for thedevelopment of a lithium secondary battery having improved C-ratecharacteristics in conjunction with maximized cell capacity.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made to solve the aboveproblems and other technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems as described above, the inventors ofthe present invention have surprisingly discovered that, when a mixtureof two or more active materials having a redox potential difference withgiven conditions is used as a cathode active material, it is possible toprepare a lithium secondary battery having improved dischargecharacteristics in a range of high-rate discharge while minimizing adead volume as described hereinbefore, and at the same time, havingincreased cell capacity via increased electrode density and loadingamounts. The present invention has been completed based on thesefindings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view of a cathode composed of one active materialhaving the same particle diameter in accordance with a conventional art;

FIG. 2 is a schematic view showing a rolled active material of FIG. 1 ata high-rolling reduction rate;

FIG. 3 is a schematic view of a cathode composed of one active materialhaving a different particle diameter in accordance with a conventionalart;

FIG. 4 is a schematic view of a cathode composed of two active materialshaving different redox potentials in accordance with one embodiment ofthe present invention;

FIG. 5 is a schematic view showing rolled active materials of FIG. 4with a high-rolling reduction rate;

FIG. 6 is a schematic view of a cathode composed of two active materialshaving different particle diameters and redox potentials in accordancewith another embodiment of the present invention;

FIG. 7 is a graph showing electric potential changes versus dischargecapacity (discharge rate) of active materials used in some experimentsof the present invention;

FIG. 8 is a comparison graph of discharge capacity corresponding to eachC-rate, obtained in Experimental Example 1 for battery cells of Examples5 and 6 and Comparative Examples 4 and 5;

FIG. 9 is a comparison graph of discharge capacity corresponding to eachC-rate, obtained in Experimental Example 2 for battery cells ofComparative Examples 6 through 8;

FIG. 10 is a comparison graph of discharge capacity corresponding toeach C-rate, obtained in Experimental Example 2 for battery cells ofExamples 7 through 9;

FIG. 11 is a comparison graph of discharge capacity corresponding toeach C-rate, obtained in Experimental Example 3 for battery cells ofExamples 10 through 12 and Comparative Examples 9 through 11;

FIG. 12 is a comparison graph of discharge capacity corresponding to therespective C-rates, obtained in Experimental Example 4 for battery cellsof Examples 6 and 13, and Comparative Examples 12 and 13;

FIG. 13 is a comparison graph of discharge capacity corresponding toeach C-rate, obtained in Experimental Example 5 for battery cells ofExamples 14 and 15;

FIG. 14 is a graph showing electric potential changes versus dischargerate of active materials used for preparing battery cells of someComparative Examples of the present invention; and

FIGS. 15 and 16 are graphs showing electric potential changes versusdischarge rate of active materials used for preparing battery cells ofsome Comparative Examples of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with an aspect of the present invention, the above andother objects can be accomplished by the provision of a cathode activematerial for a lithium secondary battery, characterized in that thecathode active material is comprised of two or more active materialshaving different redox levels so as to exert superior dischargecharacteristics in a range of high-rate discharge by sequential actionof cathode active materials in a discharge process.

As used herein, the term “redox level” refers to an electric potentialof the plateau range in the discharge process or an electric potentialat a discharge rate of approximately 50%. Preferably, the activematerials in accordance with the present invention have the redox levelin the range of 3.5 to 4.5 V. The difference of the redox level betweenactive materials will also be referred to as “a redox potentialdifference” As used herein, the phrase “sequential action of cathodeactive materials in a discharge process” means that a cathode activematerial having a relatively high redox level (hereinafter, referred toas a high-potential cathode active material) preferentially acts in thedischarge process, followed by the action of a cathode active materialhaving a relatively low redox level (hereinafter, referred to aslow-potential cathode active material).

As used herein, the term “sequential action” means that relatively largeamounts of the high-potential cathode active material preferentiallyact, but does not mean that action of low-potential cathode activematerial is initiated after all of the high-potential cathode activematerial has acted. Therefore, this term also encompasses the conditionin which the low-potential cathode active material acts in the dischargeprocess at a time point where a substantial amount, for example morethan 50%, of the high-potential cathode active materials has acted inthe discharge process.

In accordance with the present invention, when the cathode is formed ofactive materials having different redox levels, for example when theredox level of the active material A is 3.8 V and that of activematerial B is 4.0 V, the active material B does not undergooxidation-reduction in an electric potential region of 3.8 V whereoxidation-reduction of active material A takes place, and serves as anelectrolyte carrier, i.e., a void. In contrast, active material A servesas a void in 4.0 V electric potential region where oxidation-reductionof active material B takes place. As a result, substantially higherporosity is attained in the redox level regions of each active materialA and B whose oxidation-reduction occurs under the conditions in whichthe apparent porosity possessed by the electrode is generally the same.

The reason why such phenomena occur is because lithium (Li) ions shouldbe smoothly supplied during oxidation-reduction of the active materials,while a higher C-rate leads to higher consumption of lithium (Li) ionsfor the same period of time. Therefore, when one active material issingly used and sufficient porosity is not secured due to increasedrolling reduction rate of the active material, as shown in theabove-described conventional art of FIG. 2, smooth supply of lithiumions is not effected and the higher C-rate leads to rapidly decreasedcapacity and service life of the electrode active material.

In contrast, when the cathode is formed of two or more active materialshaving a predetermined redox potential difference according to thepresent invention, electrical conductivity of the electrode can beincreased even when a rolling reduction rate is increased as shown inFIG. 5, and as will be described hereinafter, it is therefore possibleto achieve increased cell capacity via improved C rate characteristicsand increased electrode density.

The above-mentioned “range of high-rate discharge” may be affected by avariety of factors and for example, may be set to a range in which asignificant decline of the discharge capacity occurs. Typically, such adischarge range may be flexibly determined depending upon supply stateof an electrolyte inside the battery. As can be seen from ExperimentalExample 6 which will be described hereinafter, the discharge range maybe set at a relatively low discharge rate in a battery having afull-cell structure in which supply of an electrolyte is limited.

Such discharge characteristics in the range of high-rate discharge willbe often referred to hereinafter as “C-rate characteristics”. Asdescribed hereinbefore, the cathode active materials in accordance withthe present invention exert superior C-rate characteristics viasequential action of each active material. The phrase “superior C-ratecharacteristics” as used herein means that the actual C-rate valuesmeasured for mixed active materials are significantly large, as comparedto calculated values (predicted values) with respect to a mixing ratioin mixed active materials, based on C-rate values measured independentlyfor each active material. Such facts are results that were completelyunpredictable prior to the present invention.

Therefore, even when the apparent porosity is lessened via high-rollingreduction rate so as to increase capacity, as shown in FIG. 5, thecathode active material in accordance with the present invention canexhibit superior C rate characteristics that were completelyunpredictable before. Even though the redox potential differences ofactive materials, which exhibit sequential action as described above andconsequently exert superior C rate characteristics, are not particularlydefined as critical values, it was confirmed, as will be seen inExperimental Examples hereinafter, that desired results are not obtainedwhen potential differences between active materials used in experimentsare less than 0.03 V.

The different active materials in accordance with the present invention,i.e. heterogeneous active materials, may be selected from activematerials represented by Formulae I through IV below. Different redoxlevels are obtained depending upon kinds of transition metals containedin each active material, which are involved in oxidation-reduction, andoxidation numbers thereof. In addition, even when the same transitionmetals take part in oxidation-reduction, active materials may exhibitdifferent redox levels, depending upon the composition and chemicalstructure thereof.

Specifically, the active materials that are used in the presentinvention may include active materials represented by Formulae I throughIV below:Li_(1+x)Co_(1−y)M_(y)O₂A_(a)  [Formula I]

wherein,

−0.2<x<0.2;

0≦y≦0.2;

0≦a≦0.05;

M is a first row transition metal such as Ni, Mn, Fe, Cr, Ti, Zn or V,Al, or Mg; and

A is an element of Group 6A or Group 7A such as S, Se, F, Cl or I.

Materials of Formula I are active materials having electrochemicalcharacteristics of Co³⁺

Co⁴⁺ oxidation-reduction in layered structures thereof.Li_(1+x)Ni_(1−y−z)M_(y)M′_(z)O₂A_(a)  [Formula II]

wherein,

−0.2<x<0.2;

0≦y≦0.2;

0≦z≦0.2;

0≦a≦0.05;

each M and M′ is independently a first row transition metal such as Co,Mn, Fe, Cr, Ti, Zn or V, Al, or Mg; and

A is an element of Group 6A or Group 7A such as S, Se, F, Cl or I.

Materials of Formula II are active materials having electrochemicalcharacteristics of Ni³⁺

Ni⁴⁺ oxidation-reduction in layered structures thereof.Li_(1+x)Ni_(1−y−z)M_(y)M′_(z)O₂A_(a)  [Formula III]

wherein,

−0.2<x<0.2;

0≦y≦0.2;

0≦z≦0.2;

0≦a≦0.05;

each M and M′ is independently a first row transition metal such as Co,Mn, Fe, Cr, Ti, Zn or V, Al, or Mg; and

A is an element of Group 6A or Group 7A such as S, Se, F, Cl or I.

Materials of Formula III are active materials having electrochemicalcharacteristics of Ni²⁺

Ni⁴⁺ oxidation-reduction in layered structures thereof.Li_(1+x)Mn_(2−y)M_(y)O₄A_(a)  [Formula IV]

wherein,

−0.2≦x≦0.2;

0≦y≦0.4;

0≦a≦0.05;

M is a first row transition metal such as Ni, Mn, Fe, Cr, Ti, Zn or V,Al, or Mg; and

A is an element of Group 6A or Group 7A such as S, Se, F, Cl or I.

Materials of Formula IV are active materials having electrochemicalcharacteristics of Mn³⁺

Mn⁴⁺ oxidation-reduction in spinel structures thereof.

In one specific embodiment, electrodes including the cathode activematerials in accordance with the present invention may be composed oftwo active materials selected from Formulae I through IV above. Specificexamples may include the following combinations, and contents of eitherof active materials in combinations may be in the range of 15 to 50%,based on the total weight of the active materials.

Active material (A): LiCoO₂ Active material (B):LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂

Active material (A): LiCoO₂ Active material (B):LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂

Active material (A): LiCoO₂ Active material (B):LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂

Active material (A): LiCoO₂ Active material (B): LiMn₂O₄

Active material (A): LiMn₂O₄ Active material (B):LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂

Active material (A): LiMn₂O₄ Active material (B):LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂

Active material (A): LiMn₂O₄ Active material (B):LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂

In particular, when the above active materials have different averageparticle diameters, it is possible to provide high-electrode densitiesand to increase loading amounts of electrodes. For example, from FIG. 6schematically showing an electrode composed of active material A havinga relatively large particle diameter and active material B having arelatively small particle diameter, it can be seen that it is possibleto increase the electrode density and electrode loading amounts whilemaintaining inherently high porosity, from the viewpoint ofcharacteristics in that the present invention uses heterogeneous activematerials.

Regarding differences in average particle diameters between activematerials, the size of active material B having a relatively smallparticle diameter may be less than 50%, preferably in the range of 10 to35%, of that of active material A having a relatively large particlediameter, upon taking into consideration actual porosity and electrodedensity. From theoretical calculation on the assumption that all of theactive materials are spherical, the particle diameter size ofsmall-particle diameter active material B capable of being filled intoempty spaces which are formed by large-particle diameter active materialA, should be less than or equal to a product from a factor of 0.225× theparticle diameter of large-particle diameter active material A. However,since active materials A and B generally are not of perfect sphericalshapes, it is possible to achieve increased density even within theabove-specified range. Absolute size differences may be preferably morethan 10 μm.

Where the electrode is composed of two active materials, the content ofactive material B having a relatively small particle diameter may bepreferably in the range of 15 to 50%, more preferably 20 to 35%, basedon the total weight of the active material mixture (A+B). Fromexperiments conducted by the present inventors, it was confirmed thataddition of less than 15% content of small-particle diameter activematerial B exhibits essentially no addition effects or has insignificanteffects on improvement of C rate characteristics. In contrast, when thecontent of small-particle diameter active material B is too high, it isdifficult to accomplish improvement of the electrode density.

For example, two active materials having different average particlediameter to each other may be used by any combination of two or moreactive materials selected from Formulae I through IV. Examples ofpreferred combinations may include, but are not limited to, thefollowing combinations. The particle diameter of active materials A isdefined as being larger than that of active materials B.

Active material (A): LiCoO₂ Active material (B):LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂

Active material (A): LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ Active material (B):LiCoO₂

Active material (A): LiCoO₂ Active material (B):LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂

Active material (A): LiNi_(0.8)Co_(0.15)Mn_(0.5)O₂ Active material (B):LiCoO₂

Active material (A): LiCoO₂ Active material (B):LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂

Active material (A): LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Active material (B):LiCoO₂

Active material (A): LiCoO₂ Active material (B): LiMn₂O₄

Active material (A): LiMn₂O₄ Active material (B): LiCoO₂

Active material (A): LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ Active material (B):LiMn₂O₄

Active material (A): LiMn₂O₄ Active material (B):LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂

Active material (A): LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂ Active material (B):LiMn₂O₄

Active material (A): LiMn₂O₄ Active material (B):LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂

In accordance with another aspect of the present invention, there isprovided a lithium secondary battery comprising the above-mentionedcathode active material. In general, the lithium secondary battery iscomprised of a cathode, an anode, a separator, and a non-aqueouselectrolyte containing a lithium salt.

The cathode is, for example, fabricated by applying a mixture of theabove-mentioned cathode active material, a conductive material and abinder to a cathode current collector, followed by drying. If desired, afiller may be added to the above mixture.

The cathode current collector is generally fabricated to have athickness of 3 to 500 μm. There is no particular limit to the cathodecurrent collector, so long as it has high conductivity without causingchemical changes in the battery. As examples of the cathode currentcollector, mention may be made of stainless steel, aluminum, nickel,titanium, sintered carbon and aluminum or stainless steel which wassurface-treated with carbon, nickel, titanium or silver. The currentcollector may be fabricated to have fine irregularities on the surfacethereof so as to enhance adhesiveness to the cathode active material. Inaddition, the current collector may take various forms including films,sheets, foils, nets, porous structures, foams and non-woven fabrics.

The conductive material utilized in the present invention is typicallyadded in an amount of 1 to 50% by weight, based on the total weight ofthe mixture including the cathode active material. There is noparticular limit to the conductive material, so long as it has suitableconductivity without causing chemical changes in the battery. Asexamples of conductive materials, mention may be made of conductivematerials, including graphite such as natural or artificial graphite;carbon blacks such as carbon black, acetylene black, Ketjen black,channel black, furnace black, lamp black and thermal black; conductivefibers such as carbon fibers and metallic fibers; metallic powders suchas carbon fluoride powder, aluminum powder and nickel powder; conductivewhiskers such as zinc oxide and potassium titanate; conductive metaloxides such as titanium oxide; and polyphenylene derivatives.

The binder is an ingredient assisting in bonding between the activematerial and conductive material, and in binding to current collectors.The binder utilized in the present invention is typically added in anamount of 1 to 50% by weight, based on the total weight of the mixtureincluding the cathode active material. As examples of the binder,mention may be made of polyvinylidene fluoride, polyvinyl alcohols,carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrollidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM),sulfonated EPDM, styrene-butadiene rubber, fluoro rubber and variouscopolymers.

The filler is an optional ingredient that inhibits cathode expansion.There is no particular limit to the filler, so long as it does not causechemical changes in the battery and is a fibrous material. As examplesof the filler, there may be used olefin polymers such as polyethyleneand polypropylene; and fibrous materials such as glass fiber and carbonfiber.

The anode is fabricated by applying an anode active material to an anodecurrent collector, followed by drying. If necessary, other components,as described above, may be further added.

The anode current collector is generally fabricated to have a thicknessof 3 to 500 μm. There is no particular limit to the anode currentcollector, so long as it has suitable conductivity without causingchemical changes in the battery. As examples of the anode currentcollector, mention may be made of copper, stainless steel, aluminum,nickel, titanium, sintered carbon, copper or stainless steel having asurface treated with carbon, nickel, titanium or silver, andaluminum-cadmium alloys. Similar to the cathode current collector, theanode current collector may also be fabricated to form fineirregularities on the surface thereof so as to enhance adhesiveness tothe anode active material. In addition, the anode current collector maytake various forms including films, sheets, foils, nets, porousstructures, foams and non-woven fabrics.

As examples of the anode active materials utilizable in the presentinvention, mention may be made of carbon such as non-graphitizing carbonand graphite-based carbon; metal composite oxides such as Li_(x)Fe₂O₃(0≦x≦1), Li_(x)WO₂ (0≦x≦1) and Sn_(x)Me_(1−x)Me′_(y)O_(z) (Me=Mn, Fe, Pbor Ge; Me′=Al, B, P or Si, a group I, II or III element of the PeriodicTable, or a halogen atom; 0<x≦1; 1≦y≦3; and 1≦z≦8); lithium metals;lithium alloys; silicon-based alloys; tin-based alloys; metal oxidessuch as SnO, SnO₂, PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO,GeO₂, Bi₂O₃, Bi₂O₄, and Bi₂O₅; conductive polymers such aspolyacetylene; and Li—Co—Ni-based materials.

The separator is disposed between the cathode and anode. As theseparator, an insulating thin film having high ion permeability andmechanical strength is used. The separator typically has a pore diameterof 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator,sheets or non-woven fabrics made of an olefin polymer such aspolypropylene and/or glass fibers or polyethylene, which have chemicalresistance and hydrophobicity, are used. When a solid electrolyte suchas a polymer is employed as the electrolyte, the solid electrolyte mayalso serve as both the separator and electrolyte.

The non-aqueous electrolyte that can be utilized in the lithiumsecondary battery of the present invention may include cyclic carbonatesand linear carbonates as a polar organic solvent. Examples of the cycliccarbonates include ethylene carbonate (EC), propylene carbonate (PC) andgamma-butyro lactone (GBL). The linear carbonate may preferably include,without limitation, at least one selected from the group consisting ofdiethylcarbonate (DEC), dimethylcarbonate (DMC), ethylmethylcarbonate(EMC) and methylpropylcarbonate (MPC). Further, the non-aqueouselectrolyte contains a lithium salt in conjunction with the carbonatecompound. Preferably, specific examples of lithium salts may be selectedfrom the group consisting of LiClO₄, LiCF₃SO₃, LiPF₆, LiBF₄, LiAsF₆ andLiN(CF₃SO₂)₂ without being limited thereto.

The lithium secondary battery of the present invention may be preparedby conventional methods known in the art, for example disposing a porousseparator between the cathode and anode, followed by introduction of anon-aqueous electrolyte.

There is no limit to shapes of the lithium secondary battery inaccordance with the present invention, and for example, mention may bemade of cylinders, squares or pouches.

EXAMPLES

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Example 1

2.5 g of an active material, which was composed of a mixture of LiCoO₂having an average particle size of 20 μm andLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ having an average particle size of 5 μm ina weight ratio of 9:1, was placed in a cylindrical mold having adiameter of 1.5 cm and was pressurized under pressure of 4000 psi for 5min. Thereafter, the thickness of the mold filled with the activematerial and the initial thickness of the empty mold prior tointroduction of the active material were respectively determined,thereby calculating the thickness of the molded active material. Inaddition, taking into account when a ratio of the active material, aconductive material and a binder was 95:2.5:2.5, based on thiscalculation result, an electrode density and changes of voids in anactual electrode were calculated. The experimental results thus obtainedare given in Table 1 below.

Example 2

An experiment was carried out in the same manner as in Example 1, exceptthat 2.5 g of an active material composed of LiCoO₂ having an averageparticle size of 20 μm and LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ having anaverage particle size of 5 μm in a weight ratio of 8.5:1.5 was added.The experimental results thus obtained are given in Table 1 below.

Example 3

An experiment was carried out in the same manner as in Example 1, exceptthat 2.5 g of an active material composed of LiCoO₂ having an averageparticle size of 20 μm and LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ having anaverage particle size of 5 μm in a weight ratio of 8:2 was added. Theexperimental results thus obtained are given in Table 1 below.

Example 4

An experiment was carried out in the same manner as in Example 1, exceptthat 2.5 g of an active material composed of LiCoO₂ having an averageparticle size of 20 μm and LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ having anaverage particle size of 5 μm in a weight ratio of 7:3 was added. Theexperimental results thus obtained are given in Table 1 below.

Comparative Example 1

An experiment was carried out in the same manner as in Example 1, exceptthat 2.5 g of LiCoO₂ having an average particle size of 20 μm was addedas an active material. The experimental results thus obtained are givenin Table 1 below.

Comparative Example 2

An experiment was carried out in the same manner as in Example 1, exceptthat 2.5 g of an active material composed of LiCoO₂ having an averageparticle size of 20 μm and LiCoO₂ having an average particle size of 5μm in a weight ratio of 9:1 was added. The experimental results thusobtained are given in Table 1 below.

Comparative Example 3

An experiment was carried out in the same manner as in Example 1, exceptthat 2.5 g of an active material composed of LiCoO₂ having an averageparticle size of 20 μm and LiCoO₂ having an average particle size of 5μm in a weight ratio of 8:2 was added. The experimental results thusobtained are given in Table 1 below. TABLE 1 Thickness Electrode densityElectrode porosity Active material (cm) (g/cc) (%) Comp. Example 1 0.393.629 18.6 Comp. Example 2 0.38 3.724 16.4 Comp. Example 3 0.379 3.73716.1 Example 1 0.384 3.685 16.9 Example 2 0.38 3.724 15.8 Example 3 0.383.724 15.6 Example 4 0.388 3.646 18.2

As can be seen from Table 1, when the active materials having adifferent particle diameter to each other were mixed (ComparativeExamples 2 and 3, and Examples 1 through 4), a density between activematerials was increased and the void was decreased, as compared to whenthe active material having the same particle diameter was used alone.

Example 5

An active material mixed in a weight ratio as in Example 3, a conductivematerial and a binder were mixed in a ratio of 95:2.5:2.5 to prepare aslurry. The slurry thus obtained was coated on aluminum (Al) foil havinga thickness of 20 μm to prepare a cathode. Thereafter, a coin-type cellwas manufactured using the thus-prepared cathode, a lithium metal as ananode, and 1M LiPF₆ in EC:EMC (1:2) as an electrolyte.

Example 6

An active material mixed in a weight ratio as in Example 4, a conductivematerial and a binder were mixed in a ratio of 95:2.5:2.5 to prepare aslurry. The slurry thus obtained was coated on aluminum (Al) foil havinga thickness of 20 μm to prepare a cathode. Thereafter, a coin-type cellwas manufactured using the thus-prepared cathode, a lithium metal as ananode, and 1M LiPF₆ in EC:EMC (1:2) as an electrolyte.

Comparative Example 4

An active material of Comparative Example 1, a conductive material and abinder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. Theslurry thus obtained was coated on aluminum (Al) foil having a thicknessof 20 μm to prepare a cathode. Thereafter, a coin-type cell wasmanufactured using the thus-prepared cathode, a lithium metal as ananode, and 1M LiPF₆ in EC:EMC (1:2) as an electrolyte.

Comparative Example 5

An active material mixed in a weight ratio as in Comparative Example 3,a conductive material and a binder were mixed in a ratio of 95:2.5:2.5to prepare a slurry. The slurry thus obtained was coated on aluminumfoil (Al) having a thickness of 20 μm to prepare a cathode. Thereafter,a coin-type cell was manufactured using the thus-prepared cathode, alithium metal as an anode, and 1M LiPF₆ in EC:EMC (1:2) as anelectrolyte.

FIG. 7 is a graph showing electric potential changes versus dischargecapacity (discharge rate) of active materials used in some experiments.As can be seen from FIG. 7, the respective active materials exhibitplateau ranges having substantially no changes of a slope at a dischargerate of about 10 to 90%. In the following experiments, redox levels ofactive materials concerned are established as the magnitude of electricpotentials at a 50% discharge rate. For example, the redox level ofLiCoO₂ is 3.92 V while that of LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ is 3.77 V,and therefore two active materials exhibit a redox potential differenceof about 0.15 V therebetween.

Experimental Example 1

For battery cells prepared in Examples 5 and 6 and battery cellsprepared in Comparative Examples 4 and 5, discharge capacity (charged at0.2 C rate) thereof was measured at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 3C rates, and a ratio of discharge capacity at the respective C-ratesrelative to 0.2 C discharge capacity was calculated. The results thusobtained are shown in FIG. 8. As shown in FIG. 8, it can be seen thatuse of two active materials having different redox potentials as inExamples 5 and 6 provides gradually better results starting from theC-rate of more than 1 C, as compared to use of one active materialhaving the same particle diameter as in Comparative Example 4 or use ofone active material having the different particle diameter as inComparative Example 5. In particular, it can be seen that an increasingratio of LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ results in superior results.

Upon considering the fact that LiCoO₂ is known to have C ratecharacteristics superior to LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ (see FIG. 7),the results of Examples 5 and 6, exhibiting improved C ratecharacteristics by addition of LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ havinginferior C rate characteristics to LiCoO₂ as contrary to theexpectation, are extraordinarily new results that were completelyunpredictable from conventional arts.

Example 7

An active material composed of a mixture of LiCoO₂ having an averageparticle size of 20 μm and LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ having anaverage particle size of 5 μm in a weight ratio of 1:1, a conductivematerial and a binder were mixed in a ratio of 95:2.5:2.5 to prepare aslurry. The slurry thus obtained was coated on aluminum foil (Al) havinga thickness of 20 μm to prepare a cathode. Thereafter, a coin-type cellwas manufactured using the thus-prepared cathode, a lithium metal as ananode, and 1M LiPF₆ in EC:EMC (1:2) as an electrolyte.

Example 8

An active material composed of a mixture ofLiNi_(0.8)Co_(0.15)Mn_(0.05)O₂ having an average particle size of 18 μnand LiMn₂O₄ having an average particle size of 5 μm in a weight ratio of7:3, a conductive material and a binder were mixed in a ratio of95:2.5:2.5 to prepare a slurry. The slurry thus obtained was coated onaluminum foil (Al) having a thickness of 20 μm to prepare a cathode.Thereafter, a coin-type cell was manufactured using the thus-preparedcathode, a lithium metal as an anode, and 1M LiPF₆ in EC:EMC (1:2) as anelectrolyte.

Example 9

An active material composed of a mixture of LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂having an average particle size of 18 μm and LiMn₂O₄ having an averageparticle size of 5 μm in a weight ratio of 7:3, a conductive materialand a binder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry.The slurry thus obtained was coated on aluminum foil (Al) having athickness of 20 μm to prepare a cathode. Thereafter, a coin-type cellwas manufactured using the thus-prepared cathode, a lithium metal as ananode, and 1M LiPF₆ in EC:EMC (1:2) as an electrolyte.

Comparative Example 6

An active material composed of LiCoO₂ having an average particle size of20 μm, a conductive material and a binder were mixed in a ratio of95:2.5:2.5 to prepare a slurry. The slurry thus obtained was coated onaluminum (Al) foil having a thickness of 20 μm to prepare a cathode.Thereafter, a coin-type cell was manufactured using the thus-preparedcathode, a lithium metal as an anode, and 1M LiPF₆ in EC:EMC (1:2) as anelectrolyte.

Comparative Example 7

An active material composed of LiNi_(0.8)Co_(0.15)Mn_(0.05)O₂ having anaverage particle size of 18 μm, a conductive material and a binder weremixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry thusobtained was coated on aluminum (Al) foil having a thickness of 20 μm toprepare a cathode. Thereafter, a coin-type cell was manufactured usingthe thus-prepared cathode, a lithium metal as an anode, and 1M LiPF₆ inEC:EMC (1:2) as an electrolyte.

Comparative Example 8

An active material composed of LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ having anaverage particle size of 20 μm, a conductive material and a binder weremixed in a ratio of 95:2.5:2.5 to prepare a slurry. The slurry thusobtained was coated on aluminum (Al) foil having a thickness of 20 μm toprepare a cathode. Thereafter, a coin-type cell was manufactured usingthe thus-prepared cathode, a lithium metal as an anode, and 1M LiPF₆ inEC:EMC (1:2) as an electrolyte.

Experimental Example 2

For respective battery cells prepared in Comparative Examples 6 through8, discharge capacity (charged at 0.2 C rate) thereof was measured atdischarge rates of 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C, and was thencalculated as a ratio relative to 0.2 C discharge capacity. The resultsthus obtained are shown in FIG. 9. In addition, for battery cellsprepared in Examples 7 through 9, discharge capacity thereof wasmeasured at discharge rates of 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C, and wasthen calculated as a ratio relative to 0.2 C discharge capacity. Theresults thus obtained are shown in FIG. 10. Similar to FIG. 8, it can beseen from two graphs that C rate characteristics were improved, in spiteof the fact that LiCoO₂ having superior C rate characteristics was mixedwith LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ having relatively poor C ratecharacteristics. Further, it can be seen thatLiNi_(0.8)Cu_(0.15)Mn_(0.05)O₂ and LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ alsoexhibit improvement in C rate characteristics, when they are used inadmixture with LiMn₂O₄ having a different redox level, as compared towhen they are used alone. For reference, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂has a redox level of 3.80 V and that of LiMn₂O₄ is 4.06 V.

Example 10

An active material mixed in a weight ratio as in Example 4, a conductivematerial and a binder were mixed in a ratio of 95:2.5:2.5 to prepare aslurry. The slurry thus obtained was coated on aluminum (Al) foil havinga thickness of 20 μm to prepare a cathode having a loading amount of 2.5mAh/cm². Thereafter, a coin-type cell was manufactured using thethus-prepared cathode, a lithium metal as an anode, and 1M LiPF₆ inEC:EMC (1:2) as an electrolyte.

Example 11

An active material mixed in a weight ratio as in Example 4, a conductivematerial and a binder were mixed in a ratio of 95:2.5:2.5 to prepare aslurry. The slurry thus obtained was coated on aluminum (Al) foil havinga thickness of 20 μm to prepare a cathode having a loading amount of 3.0mAh/cm². Thereafter, a coin-type battery cell was manufactured using thethus-prepared cathode, a lithium metal as an anode, and 1M LiPF₆ inEC:EMC (1:2) as an electrolyte.

Example 12

An active material mixed in a weight ratio as in Example 4, a conductivematerial and a binder were mixed in a ratio of 95:2.5:2.5 to prepare aslurry. The slurry thus obtained was coated on aluminum (Al) foil havinga thickness of 20 μm to prepare a cathode having a loading amount of 3.5mAh/cm². Thereafter, a coin-type battery cell was manufactured using thethus-prepared cathode, a lithium metal as an anode, and 1M LiPF₆ inEC:EMC (1.2) as an electrolyte.

Comparative Example 9

An active material of Comparative Example 1, a conductive material and abinder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. Theslurry thus obtained was coated on aluminum (Al) foil having a thicknessof 20 μm to prepare a cathode having a loading amount of 2.5 mAh/cm².Thereafter, a coin-type battery cell was manufactured using thethus-prepared cathode, a lithium metal as an anode, and 1M LiPF₆ inEC:EMC (1:2) as an electrolyte.

Comparative Example 10

An active material of Comparative Example 1, a conductive material and abinder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. Theslurry thus obtained was coated on aluminum (Al) foil having a thicknessof 20 μm to prepare a cathode having a loading amount of 3.0 mAh/cm².Thereafter, a coin-type battery cell was manufactured using thethus-prepared cathode, a lithium metal as an anode, and 1M LiPF₆ inEC:EMC (1:2) as an electrolyte.

Comparative Example 11

An active material of Comparative Example 1, a conductive material and abinder were mixed in a ratio of 95:2.5:2.5 to prepare a slurry. Theslurry thus obtained was coated on aluminum (Al) foil having a thicknessof 20 μm to prepare a cathode having a loading amount of 3.5 mAh/cm².Thereafter, a coin-type battery cell was manufactured using thethus-prepared cathode, a lithium metal as an anode, and 1M LiPF₆ inEC:EMC (1:2) as an electrolyte.

Experimental Example 3

For respective battery cells prepared in Comparative Examples 9 through11 and Examples 10 through 12, discharge capacity thereof was measuredat discharge rates of 0.1 C, 0.2 C, 0.5 C and 1 C, and was thencalculated as a ratio relative to the reference discharge capacity at0.2 C rate. The results thus obtained are shown in FIG. 11. As shown inFIG. 11, it can be seen that the cells of Comparative Example 9 andExample 10, having low-loading amounts, exhibited similar C ratecharacteristics therebetween, whereas the cells of Comparative Examples10 and 11, and Examples 111 and 12, which use a mixture of two activematerials having different redox levels with increasing loading amounts,exhibited improvement in C rate characteristics.

Example 13

A mixed active material of Example 4 (a mixture of LiCoO₂ having anaverage particle size of 20 μm and LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ havingan average particle size of 5 μm), a conductive material and a binderwere mixed in a weight ratio of 95:2.5:2.5 to prepare a slurry. Theslurry thus obtained was coated on aluminum (Al) foil having a thicknessof 20 μm to prepare a cathode with a loading amount of 2.4 mAh/cm²(based on a discharge loading). In addition, artificial graphite, aconductive material and a binder were mixed in a weight ratio of 94:1:5to prepare a slurry. The slurry thus obtained was coated on copper (Cu)foil having a thickness of 10 μm to prepare an anode with a loadingamount of 2.4 mAh/cm² (based on discharge loading). The thus-preparedcathode and anode were stacked with intercalation of a separatortherebetween, thereby preparing an electrode assembly. The electrodeassembly was built in a pouch-type case made up of an aluminum laminatesheet to which 1 M LiPF₆ impregnated in EC:EMC (1:2) as an electrolytewas then introduced, thereby preparing a pouch-type cell (full cell).

Comparative Example 12

An active material composed of LiCoO₂ having an average particle size of5 μm, a conductive material and a binder were mixed in a weight ratio of95:2.5:2.5 to prepare a slurry. The slurry thus obtained was coated onaluminum (Al) foil having a thickness of 20 μm to prepare a cathode witha loading amount of 2.4 mAh/cm² (based on discharge loading). Inaddition, artificial graphite, a conductive material and a binder weremixed in a ratio of 94:1:5 to prepare a slurry. The slurry thus obtainedwas coated on copper (Cu) foil having a thickness of 10 μm to prepare ananode with a loading amount of 2.4 mAh/cm² (based on discharge loading).The thus-prepared cathode and anode were stacked with intercalation of aseparator therebetween, thereby preparing an electrode assembly. Theelectrode assembly was built in a pouch-type case made up of an aluminumlaminate sheet to which 1 M LiPF₆ impregnated in EC:EMC (1:2) as anelectrolyte was the introduced, thereby preparing a pouch-type cell(full cell).

Comparative Example 13

An active material composed of LiCoO₂ having an average particle size of5 μm, a conductive material and a binder were mixed in a ratio of95:2.5:2.5 to prepare a slurry. The slurry thus obtained was coated onaluminum (Al) foil having a thickness of 20 μm to prepare a cathode.Thereafter, a coin cell was prepared using the thus-prepared cathode, alithium metal as an anode, and 1M LiPF₆ in EC:EMC (1:2) as anelectrolyte.

Experimental Example 4

For cells prepared in Examples 6 and 13 and cells prepared inComparative Examples 12 and 13, discharge capacity (charged at 0.2 Crate) thereof was measured at 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C rates.The results thus obtained are shown in FIG. 12. Although the cells ofExamples 6 and 13 have the same composition of active materials (using amixed active material of LiCoO₂ having an average particle size of 20 μmand LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ having an average particle size of 5μm), the cell of Example 13 is a cell having a full-cell structure whilethe cell of Example 6 is a cell having a coin-type structure. Similarly,although the cells of Comparative Examples 12 and 13 have the samecomposition of active materials (using an active material LiCoO₂ havingan average particle size of 5 μm), the cell of Comparative Example 12 isof a full-cell structure while the cell of Comparative Example 13 is ofa coin-type structure.

Generally, in active materials belonging to the same class, a smallersize of the active materials results in superior C rate characteristics.Therefore, as compared to the cell of Comparative Example 6 (see FIG. 9)using LiCoO₂ having an average particle size of 20 μm as the activematerial, the cell of Comparative Example 13 using LiCoO₂ having anaverage particle size of 5 μm as the active material, as shown in FIG.12, generally exhibit superior discharge characteristics even in therange of high-rate discharge when the cell is of a coin-type structure.

However, as discussed hereinbefore, as large consumption of theelectrolyte occurs in the range of high-rate discharge, there is atendency of sharply decreased discharge characteristics in a full-cellstructure of the cell in which the electrolyte acts as a limitingfactor, as compared to the coin type structure of the cell exhibiting nolimitation to the electrolyte. As a result, it was confirmed that thecell using 5 μm-sized LiCoO₂ having superior C rate characteristics asthe active material, when it was manufactured in the form of a full-cellstructure as in Comparative Example 13(12?), also exhibited a rapiddecrease of discharge characteristics in the range of high-ratedischarge as shown in FIG. 12.

On the other hand, it was confirmed that use of the mixed activematerial in accordance with the present invention leads to improvementin discharge characteristics even in a full-cell structure of the cellin which the electrolyte acts as a limiting factor. As discussedhereinbefore, LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ exhibits C-ratecharacteristics inferior to LiCoO₂. Whereas, it can be confirmed fromFIG. 12 that even though a cell having a full-cell structure wasmanufactured by addition of LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ to 20 μm-sizedLiCoO₂ having C-rate characteristics inferior to 5 μm-sized LiCoO₂ inExample 13, this cell exhibits superior discharge characteristics in therange of high-rate discharge, as compared to the cell of Example 6 whichwas manufactured in the coin-type structure having the same composition.

These are results that were completely unpredictable from conventionalarts. Based on these facts, it can be seen that even active materialshaving superior C-rate characteristics, but unfortunately showinglimitation of application thereof to the battery in which theelectrolyte acts as a limiting factor, can essentially overcome suchproblems via inventive constitution in accordance with the presentinvention.

Example 14

An active material mixed in a weight ratio as in Example 1, a conductivematerial and a binder were mixed in a ratio of 95:2.5:2.5 to prepare aslurry. The slurry thus obtained was coated on aluminum (Al) foil havinga thickness of 20 μm to prepare a cathode. Thereafter, a coin-type cellwas prepared using the thus-prepared cathode, a lithium metal as ananode, and 1M LiPF₆ in EC:EMC (1:2) as an electrolyte.

Example 15

An active material mixed in a weight ratio as in Example 2, a conductivematerial and a binder were mixed in a ratio of 95:2.5:2.5 to prepare aslurry. The slurry thus obtained was coated on aluminum (Al) foil havinga thickness of 20 μm to prepare a cathode. Thereafter, a coin-type cellwas prepared using the thus-prepared cathode, a lithium metal as ananode, and 1M LiPF₆ in EC:EMC (1:2) as an electrolyte.

Experimental Example 5

For cells prepared in Examples 14 and 15, experiments were carried outin the same manner as in Experimental Example 1. The results thusobtained are shown in FIG. 13. For comparison, experimental results ofExperimental Example 1 are also given in FIG. 13. As can be seen fromFIG. 13, the battery in which 10% by weight ofLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ having a particle diameter of 5 μm wasadded to LiCoO₂ having a particle diameter of 20 μm did not exhibit asignificant difference in addition effects, as compared to the cell ofComparative Example 4 in which LiCoO₂ having a particle diameter of 20μm was used alone, but addition of 15% by weight ofLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ having a particle diameter of 5 μm began toresult in pronounced improvement in C rate characteristics.

Comparative Examples 14 Through 21

Using LiMn₂O₄ (Comparative Example 14), LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂(Comparative Example 15), LiNi_(0.8)Mn_(0.1)Co_(0.1)O₂ (ComparativeExample 16), LiNi_(0.7)Mn_(0.05)Co_(0.25)O₂ (Comparative Example 17),LiNi_(0.8)Co_(0.2)O₂ (Comparative Example 18), LiNi_(0.5)Mn_(0.5)O₂(Comparative Example 19), LiNi_(0.45)Mn_(0.45)Co_(0.1)O₂ (ComparativeExample 20) and LiNi_(0.425)Mn_(0.425)Co_(0.15)O₂ (Comparative Example21), having various particle diameters as set forth in Table 2 below,the corresponding battery cells were respectively manufactured in thesame manner as in Example 13.

Examples 16 and 17

Using a mixed active material of LiMn₂O₄ having a particle diameter of15 μm and LiNi_(0.8)Mn_(0.1)Cu_(0.1)O₂ having a particle diameter of 6μm (Example 16), and a mixed active material of LiMn₂O₄ having aparticle diameter of 15 μm and LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ having aparticle diameter of 5 μm (Example 17), respectively, as set forth inTable 2 below, the corresponding cells were manufactured in the samemanner as in Example 13.

Comparative Examples 22 Through 24

Using a mixed active material of LiNi_(0.7)Mn_(0.05)Co_(0.25)O₂ having aparticle diameter of 12 μm and LiNi_(0.8)Co_(0.2)O₂ having a particlediameter of 6 μm (Comparative Example 22), a mixed active material ofLiNi_(0.425)Mn_(0.425)Co_(0.15)O₂ having a particle diameter of 6 μm andLiNi_(0.45)Mn_(0.45)Co_(0.1)O₂ having a particle diameter of 6 μm(Comparative Example 23) and a mixed active material ofLiNi_(0.425)Mn_(0.425)Co_(0.15)O₂ having a particle diameter of 6 μm andLiNi_(0.5)Mn_(0.5)O₂ having a particle diameter of 6 μm (ComparativeExample 24), respectively, as set forth in Table 2 below, thecorresponding cells were manufactured in the same manner as in Example13.

Experimental Example 6

First, electrical potential changes versus a discharge rate for LiMn₂O₄,LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂, LiNi_(0.8)Mn. Cu_(0.1)O₂,LiNi_(0.7)Mn_(0.05)Co_(0.25)O₂ and LiNi_(0.425)Mn_(0.425)Co_(0.15)O₂,which were active materials used to prepare cells of ComparativeExamples 14-17 and 21, were measured. The results thus obtained areshown in FIG. 14. As can be seen from FIG. 14, the respective activematerials exhibit different redox levels therebetween.

In addition, for LiNi_(0.7)Mn_(0.05)Co_(0.25)O₂, LiNi_(0.8)Co_(0.2)O₂,LiNi_(0.5)Mn_(0.5)O₂, LiNi_(0.45)Mn_(0.45)Co_(0.1)O₂ andLiNi_(0.425)Mn_(0.425)Co_(0.15)O₂, which were active materials used toprepare cells of Comparative Examples 22 through 24, electricalpotential changes versus a discharge rate were measured. The resultsthus obtained are shown in FIGS. 15 and 16. As can be seen from there,active materials used to prepare cells of Comparative Examples 22through 24 were composed of combinations of active materials having verysmall redox potential difference therebetween.

Meanwhile, for the respective cells prepared in Comparative Examples 14through 24 and Examples 16 and 17, discharge capacity thereof wasmeasured at 0.1 C, 0.2 C, 0.5 C, 1.0 C, 2.0 C, 3.0 C and 5.0 C rates.The results thus obtained are shown in Table 2 below. Calculated valuesgiven in Table 2 below are values in which C rates (observed values)individually measured for the respective active materials of ComparativeExamples 14 through 21 were calculated as a mixing ratio between therespective components upon constituting cathodes in Examples 16 and 17and Comparative Examples 22 through 24. TABLE 2 C-rate (150 mAh/g 1C)Materials Rate 0.1 0.2 0.5 1.0 2.0 3.0 5.0 LiMn₂O₄ (15 μm) Ob* 100.0100.2 100.1 99.6 97.0 86.4 71.5 LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ (5 μm) Ob100.0 98.0 95.2 92.4 88.8 85.1 78.2 LiNi_(0.8)Mn_(0.1)Co_(0.1)O₂ (6 μm)Ob 100.0 97.2 93.7 90.7 87.4 84.3 75.1 LiNi_(0.7)Mn_(0.05)Co_(0.25)O₂(12 μm) Ob 100.0 97.2 93.1 89.6 85.5 81.8 71.3 LiNi_(0.8)Co_(0.2)O₂ (6μm) Ob 100.0 96.7 91.8 87.2 83.4 79.2 67.1 LiNi_(0.5)Mn_(0.5)O₂ (6 μm)Ob 100.0 96.5 91.6 84.2 76.3 65.4 50.3 LiNi_(0.45)Mn_(0.45)Co_(0.1)O₂ (6μm) Ob 100.0 97.1 92.6 88.1 81.9 76.2 63.2Ni_(0.425)Mn_(0.425)CoCo_(0.15)O₂ (6 μm) Ob 100.0 97.7 93.0 88.5 82.677.9 65.7 LiMn₂O₄ (15 μm) + Ob 100.0 98.5 96.6 95.2 92.9 89.1 82.2LiNi_(0.8)Mn_(0.1)Co_(0.1)O₂ (6 μm) Th** 100.0 99.3 98.2 96.9 94.1 85.772.6 LiMn₂O₄ (15 μm) + Ob 100.0 98.9 97.4 96.0 94.0 89.0 83.3LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ (5 μm) Th 100.0 99.5 98.6 97.4 94.5 86.073.5 LiNi_(0.7)Mn_(0.05)Co_(0.25)O₂ (12 μm) + Ob 100.0 97.1 92.3 87.583.4 81.2 70.4 LiNi_(0.8)Co_(0.2)O₂ (6 μm) Th 100.0 97.1 92.7 88.9 84.981.0 70.0 LiNi_(0.425)Mn_(0.425)Co_(0.15)O₂ (6 μm) + Ob 100.0 97.3 92.487.9 81.5 76.1 62.9 LiNi_(0.45)Mn_(0.45)Co_(0.1)O₂ (6 μm) Th 100.0 97.592.9 88.4 82.4 77.4 65.0 LiNi_(0.425)Mn_(0.425)Co_(0.1) ₅O₂ (6 μm) + Ob100.0 96.6 91.2 86.0 78.5 72.4 59.7 LiNi_(0.5)Mn_(0.5)O₂ (6 μm) Th 100.097.3 92.6 87.2 80.7 74.1 61.1NoteOb*: Observed valueTh**: Theoretical value

As can be seen from Table 2, it was confirmed that cells of Examples 16and 17 in which cathodes were formed of active materials havingdifferent redox levels to each other exhibited the observed valuesgreater than the theoretical values at more than 3.0 C rate and inparticular, a higher discharge rate leads to increases in such adeviation and 5.0 C rate leads to occurrence of considerable deviation.

In contrast, it was also confirmed that even though the cathode of thecell was formed of active materials having different redox levels, therewere substantially no increases or even decreases in the observed valuesas compared to the theoretical values when the redox potentialdifference between active materials was not large as shown in FIGS. 15and 16 (Comparative Examples 22 through 24).

In conclusion, it is difficult to achieve desired effects of the presentinvention by simply mixing heterogeneous active materials and thereforeit can be seen that such combinations of heterogeneous active materialscan provide desired effects when active materials have the redoxpotential difference meeting conditions specified in the presentinvention.

INDUSTRIAL APPLICABILITY

As apparent from the above description, in accordance with the presentinvention, when a mixture of two or more active materials havingdifferent redox potentials to each other is used as a cathode activematerial and preferably the active materials have different particlediameters, it is possible to prepare a lithium secondary battery havingimproved discharge characteristics in a range of high-rate dischargewhile minimizing a dead volume, and at the same time, having increasedcell capacity via increased electrode density and electrode loadingamounts.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A cathode active material for a lithium secondary battery, comprisingtwo or more active materials having different redox levels so as toexert superior discharge characteristics in the range of high-ratedischarge by sequential action of cathode active materials in adischarge process.
 2. The cathode active material according to claim 1,wherein the redox level of the active materials is in the range of 3.5to 4.5 V.
 3. The cathode active material according to claim 1, whereinthe cathode active material is composed of two active materials.
 4. Thecathode active material according to claim 3, wherein the heterogeneousactive materials have a different average particle size so as to enhancean electrode density.
 5. The cathode active material according to claim4, wherein the size of active material B having a relatively smallparticle diameter is less than 50% of that of active material A having arelatively large particle diameter.
 6. The cathode active materialaccording to claim 5, wherein the size of active material B having arelatively small particle diameter is in the range of 10 to 35% of thatof active material A having a relatively large particle diameter.
 7. Thecathode active material according to claim 4, wherein the content of theactive material B having a relatively small particle diameter is in therange of 15 to 50% by weight.
 8. The cathode active material accordingto claim 7, wherein the content of the active material B having arelatively small particle diameter is in the range of 20 to 35% byweight.
 9. The cathode active material according to claim 1, wherein thecathode active material is composed of two or more active materialsselected from Formulae I through IV:Li_(1+x)Co_(1−y)M_(y)O₂A_(a)  (I) (wherein −0.2<x<0.2; 0≦y≦0.2;0≦a≦0.05; M is a first row transition metal such as Ni, Mn, Fe, Cr, Ti,Zn or V, Al, or Mg; and A is an element of Group 6A or Group 7A such asS, Se, F, Cl or I; and wherein the materials of Formula I are activematerials having electrochemical characteristics of Co³⁺

Co⁴⁺ oxidation-reduction in layered structures thereof);Li_(1+x)Ni_(1−y−z)M_(y)M′_(z)O₂A_(a)  (II) (wherein −0.2<x<0.2; 0≦y≦0.2;0≦z≦0.2; 0≦a≦0.05; each M and M′ is independently a first row transitionmetal such as Co, Mn, Fe, Cr, Ti, Zn or V, Al, or Mg; and A is anelement of Group 6A or Group 7A such as S, Se, F, Cl or I; and whereinthe materials of Formula II are active materials having electrochemicalcharacteristics of Ni³⁺

Ni⁴⁺ oxidation-reduction in layered structures thereof);Li_(1+x)Ni_(1−y−z)M_(y)M′_(z)O₂A_(a)  (III) (wherein −0.2<x<0.2;0≦y≦0.2; 0≦z≦0.2; 0≦a≦0.05; each M and M′ is independently a first rowtransition metal such as Co, Mn, Fe, Cr, Ti, Zn or V, Al, or Mg; and Ais an element of Group 6A or Group 7A such as S, Se, F, Cl or I; andwherein the materials of Formula III are active materials havingelectrochemical characteristics of Ni²⁺

Ni⁴⁺ oxidation-reduction in layered structures thereof); andLi_(1+x)Mn_(2−y)M_(y)O₄A_(a)  (IV) (wherein −0.2<x<0.2; 0≦y≦0.4;0≦a≦0.05; M is a first row transition metal such as Ni, Mn, Fe, Cr, Ti,Zn or V, Al, or Mg; and A is an element of Group 6A or Group 7A such asS, Se, F, Cl or I; and wherein the materials of Formula IV are activematerials having electrochemical characteristics of Mn³⁺

Mn⁴⁺ oxidation-reduction in spinel structures thereof).
 10. The cathodeactive material according to claim 9, wherein the cathode activematerial is composed of two active materials selected from Formulae Ithrough IV.
 11. The cathode active material according to claim 9,wherein the cathode active material is composed of active materialshaving different average particle size.
 12. The cathode active materialaccording to claim 9, wherein the active materials are selected from thegroup consisting of LiCoO₂, LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂,LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ andLiMn₂O₄.
 13. The cathode active material according to claim 9, whereinthe active materials are composed of the following combinations: Activematerial (A): LiCoO₂ Active material (B): LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂Active material (A): LiCoO₂ Active material (B):LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂ Active material (A): LiCoO₂ Activematerial (B): LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Active material (A): LiCoO₂Active material (B): LiMn₂O₄ Active material (A): LiMn₂O₄ Activematerial (B): LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ Active material (A): LiMn₂O₄Active material (B): LiNi_(0.7)Co_(0.25)Mn_(0.0502), or Active material(A): LiMn₂O₄ Active material (B): LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂
 14. Thecathode active material according to claim 1, wherein the activematerials are mixtures of active materials A having a relatively largeparticle diameter and active materials B having a relatively smallparticle diameter, and are selected from the following combinations:Active material (A): LiCoO₂ Active material (B):LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ Active material (A):LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ Active material (B): LiCoO₂ Active material(A): LiCoO₂ Active material (B): LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂ Activematerial (A): LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂ Active material (B): LiCoO₂Active material (A): LiCoO₂ Active material (B):LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Active material (A):LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Active material (B): LiCoO₂ Active material(A): LiCoO₂ Active material (B): LiMn₂O₄ Active material (A): LiMn₂O₄Active material (B): LiCoO₂ Active material (A):LiNi_(1/3)Mn₁₃Co_(1/3)O₂ Active material (B): LiMn₂O₄ Active material(A): LiMn₂O₄ Active material (B): LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ Activematerial (A): LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂ Active material (B):LiMn₂O₄ Active material (A): LiMn₂O₄ Active material (B):LiNi_(0.7)Co_(0.25)Mn_(0.05)O₂ Active material (A): LiMn₂O₄ Activematerial (B): LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂, or Active material (A):LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ Active material (B): LiMn₂O₄
 15. Anelectrode comprising a cathode active material as set forth in claim 1.16. A lithium secondary battery comprising an electrode of claim
 15. 17.The lithium secondary battery according to claim 16, wherein the batteryis a secondary battery in which the electrolyte acts as a limitingfactor in the range of high-rate discharge.